Radiation curable materials have penetrated several areas of commerce due to their characteristic rapid processing speeds. One class of radiation curable materials are converted from a liquid to a solid upon exposure to energy from the ultraviolet portion of the electromagnetic spectrum. Such materials are commonly referred to as UV curable materials. Examples of such materials include UV curable optical fiber coatings, optical fiber cabling materials, adhesives, floor coatings, wood coatings, and metal beverage can coatings. Another class of radiation curable materials are converted from a liquid to a solid upon exposure to visible light. Such materials are commonly referred to as visible light curable materials. Examples of such materials include optical fiber coatings, pigmented inks, and adhesives.
A characteristic of all radiation curable materials is that their extent of cure is dependent upon the amount of exposed curing radiation, referred to as dose. Cure dose is a variable which is sought to be carefully controlled during processing of radiation curable materials in order to ensure complete cure of the material. Some general methods currently used to control radiation curing dose include:
1. Careful control of processing line speed;
2. Consistent exposure time for fixed substrates;
3. Output power meters for the curing radiation sources;
4. Preventive maintenance of curing radiation source reflector systems;
5. Routine replacement of curing radiation sources; and
6. Off-line, destructive quality control inspection of cured article to gauge the ultimate degree of cure achieved.
One growing area of use for curable materials is as coatings for optical fibers. In the manufacture of optical fiber, a perform is suspended vertically and moved into a furnace. Optical fiber is drawn from the perform and afterwards is provided with one or more layers of various liquid coating material. In general, the liquid coating material is a curable coating material and typically it is an ultraviolet (UV) light energy curable material.
Curing of the coating materials is accomplished by moving successive lengths of the optical fiber having the coating material thereon through a center tube which is disposed within an elliptically shaped housing as depicted in U.S. Pat. No. 5,000,772. At one loci of the elliptical shape is disposed a curing lamp; at the other, the center tube. The elliptical housing has reflective surfaces for reflecting curing energy toward engagement with the optical fiber.
Furthermore, a quartz center tube is sometimes used to maintain an oxygen-free atmosphere (N.sub.2) for a high degree of cure of the secondary or exterior coating layer.
Disadvantageously, the interior surface of the center tube is often adiabatic. Furthermore, the heat of polymerization of the coating material and the absorption of radiation from the UV lamps may heat the fiber temperature to about 130.degree. C. or higher. Given that many coating materials devolitize at temperatures greater than about 90.degree. C., deposits of volatiles form on the inner surface of the center tube even during normal usage.
Notwithstanding the rapid flow of nitrogen gas through the center tube, there normally are sufficient contaminants deposited within about eight hours of operation of the draw line to cause about 30-40% of the ultraviolet (UV) radiation incident to the center tube surface to be absorbed instead of being transmitted toward the optical fiber as desired. This attenuation decreases the degree of cure of the optical fiber coating material, particularly as the rate of reaction increases with the UV dose. Accordingly, the center tubes must be replaced on a regular basis.
Presently, it is typical practice after the fiber has been drawn to measure the degree of cure with an off-line pullout test. In such a pullout test, the force required to pull a 1 cm length of optical fiber out of its coating is measured. This existing off-line technique offers no real-time feedback as to the overall effectiveness of the cure. Furthermore, the existing pull-out test provides no ability to isolate any particular deficiencies which may exist in various stages or components of the curing system.
One other technique presently known to monitor the degree of cure of a coating material is disclosed in commonly assigned U.S. Pat. No. 5,037,763 which issued in the name of J. R. Petisce on Aug. 6, 1991. This patent facilitates the in-line monitoring of the degree of cure by including a probe within the coating material. The probe comprises a material which emits light subsequent to being promoted to an excited electronic state.
What is sought after and what seemingly is not available in the prior art is an on-line monitoring system which may be used to continuously monitor the overall curing effectiveness of the curing system.